Method and apparatus for sampling well fluids



Aug. 30,1960 I A. w. sMlTH METHOD AND APPARA'III'US FOR SAMPLING WELLFLUIDS Filed March 15, 1958 5 Sheets-Sheet 1 INVENTOR. Aer/{0e IL SM/IHBYZ g 1 Q4 METHOD'AND 'APYPARATUSJFOR SAMPLING WELL FLUIDS v Filed Mal-c13, 1958' SSheets-Sheet 2.

nite States Patent METHOD AYD APPARATUS FOR SANIPLING WELL FLUIDS ArthurW. Smith, Midland, Tex., assignor to Aircushion Patents Corporation,Bakersfield, Calif., a corporation of California Filed lVlar. 13, 1958,Ser. No. 721,247

8 Claims. (Cl. 166-3) The present invention relates generally to theproduction of petroleum and more particularly to a new and improvedmethod and apparatus for sampling well fluids, such sampling beinggenerally termed formation testing.

in formation testing a cased or uncased well, formation tester apparatusincluding a packer is lowered into the well bore on a pipe string suchas a drill stem. The packer when properly seated against the wall of thewell bore seals off the test interval below the packer from thehydrostatic pressure of the overlying column of drilling fluid and/ orwell bore fluid in the well. Valve means in the formation tester is thenopened so as to place the packedoif test interval in communication withthe interior of the pipe string. With the interior of the pipe string atatmospheric pressure, fluid from the test interval is then permitted toflow from the formation below the packer into the pipe string. Where thepressure dilferential between the test fluid and the interior of thepipe string is high, the well bore fluid will tend to move through theformation tester and into the pipe string at a high velocity. This canreadily result in caving or collapsing of the well bore adjacent theformation tester thereby causing the tester to become stuck in the wellbore. An expensive and time-consuming fishing operation is then requiredin order to retrieve the formation tester. In some cases such fishingoperations are unsuccessful. Moreover, the stratum being sampled may Ieasily be damaged under such conditions. It is also common for the portsof the formation tester and/ or its associated equipment to becomeclogged where these high pressure differentials are encountered.

In order to overcome such disadvantages, it has been heretofore proposedto reduce such pressure differential when the formation tester is openedby filling the pipe string above the formation tester with water ordrilling fluid. This is generally termed a water cushion operation. Inthe use of such cushion an attempt is made to employ suflicient liquidto provide a pressureslightly lower than the expected pressure of thewell bore fluid in the test interval. Where too much water is employed adry run will result. If however, an insuflicient amount is used, theaforementioned disadvantages resulting from a high pressure differentialwill result.

A further important reason for the use of a water cushion is that itprevents theinternal collapse of the pipe string during a formationtesting operation. In this regard, if the interior of the pipe string ismaintained at atmospheric pressure it is subject to being collapsedunder the influence of high inwardly directed forces produced by highWell pressures and the tension to which it is subjected as it is loweredand raised within the well bore. Such collapse can be avoided by fillingthe pipe with water or drilling fluid.

It is an object of the invention to provide a method and apparatus ofsampling well fluids which provide all the advantagesof 'a successfulwater cushion and eliminates the disadvantages inherent in the use ofsuch water cushion.

Another very important object of the present invention is to provide amethod and apparatus which will restrain collapse of the pipe stringduring a formation testing operation.

' Another object of the invention is to provide a method and apparatusof the aforedescribed nature which may be employed with conventionalformation testing equipment.

A further object of the invention is to provide a method and apparatusfor sampling well fluids which permits the well fluids to reach thesurface in an uncontaminated condition. This is an important advantageover the use of the aforedescribed water cushion since it is extremelydiflicult to obtain an uncontaminated test sample where the latter mustflow through either water or drilling fluid.

Other objects and advantages of the present invention will becomeapparent from the following detailed description when taken inconjunction with the appended drawings wherein:

Figures 1 through 4 are diagrammatic illustrations showing the mode ofoperation of the preferred method and apparatus of the present inventionin formation testing a well bore;

Figure 1a is a side elevational view particularly showing a conventionalformation tester utilized with said method and operation;

Figure 5 is an enlarged, fragmentary, central vertical sectional viewshowing a loading sub used in said method and apparatus;

Figure 6 is a horizontal sectional view taken on line 6-6 of Figure 5;

Figure 7 is a view similar to Figure 6 but showing the parts of saidloading sub in a diiferent position;

Figure 8 is an enlarged, fragmentary, central vertical sectional viewshowing a control valve used in said method and apparatus;

Figure 8a is a fragmentary, central vertical sectional view of aconventional one-Way valve used with said apparatus, said view being inenlarged scale; and

Figure 9 is a fragmentary view similar to Figure 8, but showing theparts of said control valve arranged in an open position.

Referring to the drawings and particularly Figures 1 through 4 thereof,the method and apparatus of the present invention is employed inconjunction with a conventional formation tester F carried at the lowerportion 16 of a pipe string P so as to be lowered into a well bore 18.Manipulation of the pipe string P is effected by conventional equipment(not shown). A loading sub S to be fully described hereinafter isconnected to the upper end of lower pipe portion 16. This loading sub Sincludes gas-receiving means 26. At a point spaced above the loading subS, there is disposed in the pipe string a control valve V, the detailsof which are to be fully described hereinafter.

-In carrying out the method of the present invention, the control valveV is connected either to the upper end of a length of pipe 26 having itslower end connected to the loading sub S, or alternatively to the upperend of the loading sub S. Next, as indicated in Figure 1, a suitable gaswill be introduced into the loading sub S through the gas-receivingmeans 29 thereof so as to pressurize the lower pipe string portion 16 toa desired value. The use of an inert gas is preferred because it cuts toa minimum the fire hazard present at the well head and even within thewell itself. Nitrogen, an inert gas is particularly desired because ofits chemical inertness in reference to gases present in earthformations, giving a much better formation gas sample at the casing headduring subsequent for- 3 1 mation testing operations. Sufficientnitrogen will be forced through the gas-receiving means 20, as forexample by conducting gas into the gas-receiving means from banks ofcylinders (not shown) in which it may be conveniently transported to thewell as to pressurize the lower portion 16 of the pipe string P to adesired pressure. Thereafter, as indicated in Figure 2; additionallengths 28 of pipe will be connected to the upper end of the controlvalve V so as to lower the pipe string P within the well bore until theperforated anchor 15 of the formation tester F is disposed adjacent thetest interval zone 32 to be tested.

Next, referring to Figure 3, the packer 13 of'the formation tester F isseated in the well bore 18 above the test interval 32. The formationtester F willthen be opened so as to admit well bore fluids intothe'lower portion 16 of the pipe string P. A specific example of oneform of formation tester which may be employed in connection withthepresent invention is shown in US. Patent No. r

lower pipe string portion under the influence of high inwardly-directedforces produced by high well pressures and/or the tension to which thepipe string is subjected while it is being vertically manipulated withinthe well bore 18. The precise pressure of the gas in the lower pipestring portion will vary according to the various conditions encounteredfrom'well to well.

Referring now to Figure 4, the formation fluid from the test interval 32will enter the opened formation tester F and the lower portion 16 of thepipe string P gradually V and slowly, thereby preventing'caving,channeling or other damage to the formation inasmuch as the'pressuredifferential between the test interval 32 and the interior of the lowerpipe string portion 16 will be within safe limits. When the pressure ofthis incoming formation fluid rises above a predetermined value, thecontrol valve V will open in a manner to be fully described hereinafter,so as to permit such formation fluid to rise toward the earths surfacethrough the upper portion of the pipe string P. The control valve Vwill, however, automatically 7 close when thepressure within the lowerpipe string portion 16 falls below such predetermined value. 'Hence, thelower pipe string portion 16 will remain pressurized at thepredetermined value and thus resistant against collapsing, even whilethe pipe string P is raised to the surface at the conclusion of theformation testing operation.

Referring now to Figures 5, 6 and 7 there is shown a preferred form ofloadingsub S which may be employed in carrying out the presentinvention. The loading sub S includes a tubular body 40 having anoutside diameter approximating that of the pipe string P. The lowerportion of the body 40-is formed with an externally threaded malecoupling member 425adapted to receive a complementary internallythreaded female member 44 formed on the. upper end of the pipe section16; The'upper portion of the body 40 is formed-with an internallythreaded female coupling member 46 that receives a complementary malecoupling member 48 formed'on the lower end of the upper pipe section 26.Y a

- The aforementioned gas-receiving means 20 includes a transverselyextending bore 50 formed in the intermediate portion of the body 40 soas to intersect one side of the gas-receiving bore 52 of the loadingsub, as particularly shown in Figure 6. The inlet portion 54 of thegas-receiving bore 50v is of reduced diameter and is internally threadedso as to receive a removable plug 56. The opposite end of thegas-receiving bore 50 is likewise internally threaded so as to receive avalve support sleeve 58. The

-'a plurality of'ports 125.

4 valve support sleeve 58 includes a coaxial bore 60 that slidablycarries the stem 62 of a poppet valve 64. The head 66 of this poppetvalve 64 is adapted to engage a complementary seat 68 formed at thejunction of the bore 50 and its inlet portion 54. The opposite end ofthe stem 62 is externally threaded, as indicated at 70. Preferably, aseal ring 72 will be provided in the valve support sleeve 58 so as toengage the valve stem 62.. The valve support sleeve 58 includes acoaxial internally threaded cavity 74. This cavity 74 is adapted toreceive a removable plug 76.

Referring now to Figure 7, when itis desired to force gas into theloading sub S, the plugs 56 and 76 are removed from their respectivebores. The threads 70 of the valve stem 62 are engaged by complementarythreads formed within a blind bore 80 disposed at the free end of agenerally T-shaped loading tool 82. During the gas loading operation thepoppet valve 64 will be maintained in' its position of Figure 7. Whenthe desired volume of gas has been forced into the loading sub S, theloading tool 82 will be employed to force the head 66 of the poppetvalve 64 into sealing engagement with the seat 68. Thereafter, thethreads formed within the blind bore 70 of the unloading tool 82are'unscrewed from the threads 70 of the valve stem 62. The pressure ofthe gas will then maintain the valve head 66 in tight sealing engagementwith the seat 68. The plug 76 will likewise be reinserted within thecavity 74 of the valve support sleeve 58 so as to engage and therebypositively prevent inadvertent unseating of the poppet valve 64.Finally, the plug 56 will be reinstalled within the inlet portion 54 ofthe gas-receiving bore 50. The various parts of the gas-receiving means20 will then remain closed so as to prevent escape of gas despite anyrough handling of. the loading sub S. While the aforedescribed loadingsub S has proven to be particularly effective, it should be clearlyunderstood that various other arrangements may be provided forpressurizing the interior of the pipe string P below the control valveV.

Referring now to Figures 8 and 9, there is shown a preferred form ofcontrol valve V which may be employed in carrying out the presentinvention. This control valve V includes a'tubular body formed at itslower end'with an externally threaded male section 92 adapted 'to bethreadably received .by a complementary internally threaded femalesection 94 on the upper end of the pipe length 26. The lower portion ofthe sub body 90 is formed with a first counterbore 98 of. largerdiameter than its main bore 100. The lower end of counterbore 98 mergesinto a slightly larger second counterbore 102. The upper counterbore 98longitudinally slidably receives the upper portion of a mountingcylinder, generally designated 104. The lower portion of this mountingcylinder 104 is externally threaded .as indicated at 106 so as toreceive the internally threaded upper end of a coaxial gas tubegenerally designated 108. Interposed between the lower portion of themounting cylinder 104 and the upper portion of the gas tube 108'is avalve seat member, generally designated 110.-

This valve seat element 110 is coaxially formed at its lower portionwith a downwardly-facing valve seat 112. The upper end of this valveseat 112 merges into a coaxial vertical bore 114. This bore 114,constitutes an orifice with respect to the upper end of the gas tube108. A plurality of radially extending passages 116 connect the lowerportion of the bore 114 with the interior 118 of the mounting cylinder104. The upper portion of the valve seat member 110 is formed with anupstanding balance tube 120 through which extendsbore 114 with the upperend of this bore being closed by an integral cap 122. 7 V

The upper portion of the gas tube 108 is formed with a counterbore 124of larger diameter than the inner periphery 126 of the major portion ofthis gas tube. The upper portion of the' gas tube is also formed withCommunication between the annular space 128 separating the exterior ofthe gas tube 108 and the interior of the pipe length 26 and the interior118 of the mounting cylinder 104 through ports 125 is controlled bymeans of an integral, vertically movable pressure-release member,generally designated 130. This pressure release member 130 includes ablocking element 132 formed at its intermediate portion and having anannular sealing surface 134 adapted to engage the valve seat 112 ofvalve seat member 110. The lower end of this blocking element mergesinto a main piston 136, the latter being slidably disposed within thecounterbore 124 of the gas tube 108. The upper end of the pressurerelease member 130 is formed with a balancing piston 138 that isslidably arranged within the balance tube 120. The balancing piston 138and the blocking element 132 are rigidly interconnected by a neck 140.The upper portion of the blocking element 132 above sealing surface 134is slidably disposed within the lower portion of the bore 114.Conventional O-rings may be provided for effecting sealing between thepressurerelease member 130 and the valve seat member 110 and gas tube108, as shown in Figure 8. The lower end of the gas tube 108 is closedby a disc 142, this disc 142 being securely held in place as by Welding.The disc 142 is coaxially formed with an internally threaded bore 144.This bore 144 receives a complementary, externally threaded plug 146.The upper end of the bore 144 merges into a smaller counterbore 147wherein is aflixed a conventional one-way valve fitting 148. The detailsof such valve fitting are shown in Figure 81:.

Above its threaded lower portion, the mounting cylinder 104 is formedwith a radial enlargement 150. This enlargement 150 is slidably disposedwithin the lower counterbore 102 of the body 90. An O-ring or othersealing means may be provided between the extension 150 and thecounterbore 102, as indicated in Figure 8. The upper portion of themounting cylinder 104 is formed with a pair of radially inwardlydirected bores 152. These bores 152 each receive a shear pin 154. Theintermediate portion of each shear pin 154 is disposed within a radialbore 156 formed in the body 90 outwardly of the upper counterbore 98.The radially outer end of each bore 156 merges into an internallythreaded passage 158. The shear pins 154 are each held in place by anexternally threaded plug 160 disposed within threaded passage 158. Thisarrangement makes it possible to effect the downward removal of themounting cylinder 104 and the gas tube 108 should such removal becomenecessary during a formation testing operation. The shear pins 154 canbe broken, as by dropping a suitable go-devil down the pipe string, orby applying sutficient downward pump pressure against the control valveV.

The upper end of the body 90 is formed with a coupling member (notshown) permitting it to be connected to the lower end of pipe length 28.In the operation of the aforedescribed control valve V, prior to thetime this valve is connected to the lower end of the pipe length 28, theinterior of the gas tube 108 will have been filled with a suitable gas,preferably nitrogen. Such filling is effected by removing the plug 146from the disc 142. Thereafter, a conventional gas filling fitting (notshown) is secured to the disc 142 and gas is forced through the fitting148. The interior of the gas tube 108 is thus pressurized to apredetermined value. This predetermined value will be that at which itis desired to have the pressure release member 130 move downwardly fromits closed position of Figure 8 to its open position of Figure 9. Whenarranged in its open position of Figure 9, the pressure release member130 will permit formation fluid entering the formation tester F from thetest interval 32 to flow upwardly through the annular space 128,inwardly through ports 125 and upwardly through the lower end of bore ororifice 114 and passages 116 into the interior of the mounting cylinder118. As will be apparent, the interior 118 of the mount- '8 ing cylinderis in communication with the interior of the pipe string P above thecontrol valve V.

It should be particularly noted that the upwardly-facing surfaces of themain piston 136 are preferably equal to the downwardly-facing areathereof, the latter area being exposed to the pressure within thepressurized gas tube 108. The aforementioned upwardly-facing surfacesare exposed to the pressure within the space 128 separating the gas tube108 and the pipe section 26. Accordingly, the vertically directed forceexerted against the pressure release member at any given moment will beequal to the pressure differential between the interior of the gas tube108 and the space 128. As noted hereinbefore, the space 128 is initiallypressurized with nitrogen by means of the loading sub S to .a desiredvalue. The interior of the gas tube 108 will be pressurized at a highervalue. Hence, the pressure release member 130 will remain in its closedposition of Figure 8 until such time as the pressure within the space128 exceeds that within the gas tube 108. The pressure release member130 will then be moved downwardly towards its open position of Figure 9.In order to provide communication between the space 128 and theupwardly-facing area of the pressure release member 130, the uppersurface of the balance cylinder 130 is placed in communication with thespace 128 by means of a vertical passage 164 and a horizontal passage166, the latter passage being formed through the blocking element 132and intersecting the lower end of the vertical passage 164.

With continued reference to Figure 9, when the formation tester F isopened, the formation fluid from the test interval 32 will flow upwardlyinto the annular space 128. At such time as the pressure existing withinannular space 128 exceeds the pressure at which the interior of the gastube 108 is pressurized, the pressure release member 130 will be moveddownwardly to its open position of this figure. Accordingly, theformation fluid will be permitted to rise toward the earth's surfacethrough the upper portion of the pipe string P. When, however, thepressure of this formation fluid falls below that at which the gas tube108 is pressurized, the pressure release member 130 will automaticallybe moved upwardly until its sealing surface 134 again engages the valveseat 112. Communication will then be cut oif between the annular space128 and the interior of the pipe string P above the control valve V. Itwill therefore be apparent that the interior of the lower pipe stringportion 16 will always be maintained at a sufliciently high pressure torestrain collapse thereof. This becomes particularly important when thepacker 30 of the formation tester F is unseated and the pipe string israised upwardly thereby placing its lower portion under considerabletension. In this regard, the formation tester F should be closed tocommunication with the test interval 32 (in a conventional manner)before the pipe string P is withdrawn from the well bore.

It should be noted that various other types of control valves may beemployed in carrying out the present invention other than the preferredform shown in detail in Figures 8 and 9. It should also be noted thatthe method and apparatus of the present invention may be successfullyoperated utilizing various forms of formation testers and packers otherthan the types shown and described hereinabove.

Various other modifications and changes may be made with respect to theforegoing description without departing from the spirit of the inventionor the scope of the following claims.

I claim.

1. A method of testing a formation penetrated by a well, comprising:providing a conduit having a valved tester and a packer at the lower endthereof; pressuring the interior of a portion of said conduit above saidtester but below the earths surface by introducing gas under apredetermined pressure into said portion, said pressure tion while thelatter is in said well; maintaining said conduit and tester sealed fromfluids present in said well until saidpacker is set; setting said packerwhile said conduit and tester are sealed to remove the hydrostaticpressure of overlying fluid in said well from the formation to betested; providing an opening in said tester when said packer is set topermit the flow of fluid from said formation through said opening intosaid pressurized conduit portion; and effecting communication betweensaid pressurized conduit portion and the interior of said conduitthereabove to permit a rise' of said formation fluid through saidopening and into the interior of said conduit above 8 with said methodincluding the steps of; extending said pipestring downwardly into saidwar with said packer and formation tester at the lower part thereof;selecting a gas that is substantially chemically inert with respect tofluids found in said formation; pressuring the interior 7 of a portionof said pipe string above said tester but besaid pressurized conduitportion only when the fluid pressure within said pressurized conduitportion rises above said predetermined pressure whereby said pressurizedconduit portion will remain resistant against collapsing during saidtesting. 7

2. A method of testing a formation penetrated by a well, comprising:providing a conduit having a valved tester and a packer at the lower endthereof; selecting a gas that is substantially chemically inert withrespect to fluids found in said formation; pressuring the interior of aportion of said conduit above said tester but below the earths surfaceby introducing said gas under a predetermined pressure into saidportion, said pressure being high enough to prevent inward collapse'ofsaid portion while the latter is in said well; maintaining said conduitand tester sealed from fluids present in said well until said packer isset; setting said packer while said conduit and tester are sealed toremove the hydrostatic pressure of overlying fluid in said well from theformation to be tested; providing an opening in said'tester when saidpacker is set to permit the flow of fluid from said formation throughsaid opening into said pressurized conduit portion; and effectingcommunication between said pressurized conduit portion and the interiorof said conduit thereabove to permit a rise of said formation fluidthrough said opening and into the interior of said conduit above saidpressurized conduit portion only when the fluid pressure within saidpressurized conduit portion rises above said predetermined pressurewhereby said pressurized conduit portion will remain resistant againstcollapsing during said testing. 7

3. A method of testing a formation penetrated by a well, said methodutilizing a pipe string and a packer and formation tester at the lowerpart of said pipe string, with said method including the stepsof:extending said pipe string downwardly into said well with said packerand formation tester .at the lower part thereof; pressuring the interiorof. a portion of said pipe string above said tester but below the earthssurface by introducing gas under a predetermined pressure into saidportion, said pressure being high enough to prevent inward collapse ofsaid portion while the latter is in said well; maintaining said pipestring and tester sealed from fluids present in said Well until saidpacker is set; setting said packer while said pipe string and tester aresealed to remove the hydrostatic pressure of overlying fluid. in

'said well from the formation to be tested; opening said the formationfluid through said opening into the intenor of said pipe string abovesaid pressurized pipe string portion only when the fluid pressure withinsaid pressurized pipe string portion rises above said predeterminedpressurewhereby said pressurized pipe string said testing. V

4. A method of testing a formation penetrated by a well, said methodutilizing a pipe string and a packer -andforrnation tester at the lowerpart of said pipe string,

low the earths surface by introducing said gas under a predeterminedpressure into said portiomsaid pressure being high enough to preventinward'collapse of said portion while the latter is in said well;maintaining said pipe string and tester sealed from. fluids present insaid well until. said packer is set; setting said packer while said pipestring and tester are sealed to remove the hydrostatic pressure ofoverlying fluid, in said. well from the formationto be tested; openingsaid, tester when said packer is set to permitttheflow offluid from saidformation through said opening into said pressurized pipe stringportion; and eifecting communication between said pressurized pipestring portion and the interior of said pipe string thereabove'to permitarise of the formation fluid through said opening into the interior ofsaid pipe string above said pressurized pipe string portion only whenthe fluid pressure Within said pressurized pipe string portion risesabove said predetermined pressure whereby said pressurized pipe stringportion will remain resistant against collapsing during said testing. I

5. A method of testing a formation penetrated by a well, comprising:providing a conduit having a valved tester and apacker at the lower endthereof;-pressuring the interior of a portion of said conduit above saidtester but below the earths surfaceby introducing gas under Y apredetermined pressure intosaidportion, said pressure portion willremain resistant against collapsing during 7 being high enough torestrain inward collapse of said porwhen said packer is set to permitthe flow of fluid from said formation through said opening into saidpressurized conduit portion; effecting communication between saidpressurized conduit portion and the interior of said conduit thereaboveto permit arise of said formation fluid through said opening and intothe interior of said conduit above said pressurized conduit portion onlywhen the fluid pressure within said pressurized conduit portion risesabove said predetermined. pressure whereby said pressurized conduitportion will remain resistant against collapsing during said testing;and finally closing said opening and withdrawing said conduit upwardlythrough said well while continuing to maintain said pressurized conduitportion pressurized at a minimum value equal to substantially saidpredetermined pressure. i

6. A method of testing a formation penetrated by a well, comprising:providing a conduit having a valved 'tester and a packer at the lowerend thereof; selecting a gas that is substantially chemically inert withrespect to fluids found in said formation; pressuring the interior of aportion of said conduit above said tester but below the earths surfacebyin troducing said gas under a predetermined'pressure into saidportion, said pressure being high enough to restrain inward collapse ofsaid portion while the latter is in said well; maintaining said conduitand tester sealed from fluids present in said well until said packer isset; setting said packer while said conduit and tester are sealed toremove the hydrostatic pressure of overlying fluid in said well from theformation to be tested; providing an opening in said tester when saidpacker is set to permit the flow of fluid from said formation throughsaid opening into said pressurized conduit portion; eflectingcommunication between said pressurized conduit portion and theinteriorof said conduit thereabove to permit a rise of said formation fluidthrough said opening-and into the interior of said conduit above saidpressurized conduit portion only when the fluid pressure within saidpressurized conduit portion rises above said predetermined pressurewhereby said pressurized conduit portion will remain resistant againstcollapsing during said testing; and finally closing said opening andwithdrawing said conduit upwardly through said well while continuing tomaintain said pressurized conduit portion pressurized at a minimum valueequal to substantially said predetermined pressure.

7. Apparatus for use with a pipe string that extends to the earthssurface in testing a well formation, said apparatus comprising: aformation tester carried at the lower part of said pipe string; acontrol valve disposed in the lower portion of said pipe string abovesaid formation tester, said control valve having a body formed with apassage permitting fluid communication through the interior of said pipestring, a blocking element in said body movable between a first positionwherein it blocks fluid flow through said passage and a second positionwherein it permits fluid flow through said passage, and biasing means insaid body normally urging said blocking element towards said firstposition with a desired force, said biasing means permitting saidblocking element to be moved to said second position when the fluidpressure within said lower pipe string portion exceeds a predeterminedvalue whereby formation fluid entering said formation tester may flowupwardly through said control valve passage and pipe string to theearths surface; and a loading member interposed in said pipe stringbelow said control valve, with the interior of said loading member beingin communication with the interior of said pipe string below saidcontrol valve, said loading member having gas-receiving means to admit agas under superatmospheric pressure into the interior of said loadingmember before said formation tester is placed in communication with saidwell formation.

8. Apparatus for use with a pipe string that extends to the earthssurface in testing a well, comprising: a. formation tester carried atthe lower end of said pipe string; acontrol valve member'having a bodyformed with a passage; means on said body for mounting said body in thelower portion of said pipe string above said formation tester; valvemeans in said body controlling the flow of fluid through said passageand including a pressure release member movable between a closedposition and an open position, said pressure release member being formedwith a surface exposed to the interior of the lower portion of said pipestring above said formation tester; biasing means in said body urgingsaid pressure release member towards its closed position, the force ofsaid biasing means being temporarily overcome when said formation testeris opened and the pressure within said lower pipe string portion exceedsapredetermined value so as to permit said pressure release member to bemoved towards its open position and efiect temporary communicationbetween said lower pipe string portion and the remainder of said pipestring whereby formation fluid entering said formation tester may flowupwardly through said control valve passageand pipe string to the earthssurface; and a loading member disposable in said pipe string below saidcontrol valve, said loading member having gas-receiving means foradmitting gas under superatmospheric pressure into the interior of saidlower pipe string portion before said formation tester is placed incommunication with said well formation.

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